1. Field of the Invention
The present invention generally relates to communication systems, and more particularly, to a system and method for minimizing signal distortion in both the plain old telephone system (POTS) frequency band and in the asynchronous digital subscriber line (ADSL) upstream and downstream frequency bands in communication systems capable of concurrent communications in both the POTS and the ADSL frequencies.
2. Discussion of the Related Art
In recent years, telephone communication systems have expanded from traditional POTS communications to include high-speed data communications as well. As is known, POTS communications include the transmission of voice information, control signals, public switched telephone network (PSTN) information, as well as, information from ancillary equipment in analog form (i.e., from computer modems and facsimile machines) transmitted in the POTS bandwidth.
Prompted largely by the desire of large businesses to reliably transfer information over a broadband network, telecommunications service providers have employed discrete multi-tone, hereinafter DMT, systems to provide a plethora of interactive multi-media digital signals over the same existing POTS twisted-pair lines. DMT systems, by nature of their distribution across multiple frequency bands, are capable of retuning devices to optimize data transfer for changing line conditions. DMT devices selectively transfer bits from the data stream in those discrete frequency bands that are uncorrupted from amplitude modulation radio interference and unaffected by phone system bridge taps, thereby tuning, or maximizing performance under changing conditions.
The provision of digital subscriber lines (DSL) systems to customer premises has proliferated over recent years. DSL communications systems use the DMT standard and are interactive two-way communications systems. Since DSL signals are transmitted in a higher frequency band than that of the POTS frequency band, transmitting signals from both the POTS and DSL frequency bands over the same twisted-pair telephone line (even at the same time), generally is not a problem. Specifically, the POTS frequency band is generally defined from 0 Hz to 4 kHz, while DSL frequency bands are generally defined by a lower cutoff frequency of approximately 26 kHz, and an upper cutoff frequency of approximately 1 MHz.
In the past, a combination of circuits termed hybrids and POTS splitters have served to buffer DSL equipment from distortion and interference introduced in the DSL frequency bands from the lower frequency POTS equipment. In a DMT-G.Lite standard configuration, the POTS splitter is no longer present. As a result, DSL equipment operated on the same twisted-pair phone line that is being used to deliver POTS service is subject to distortion due to the presence of nonlinear electronic devices in the POTS equipment. The presence of nonlinear devices in telephone sets, i.e., voltage limiters, amplifiers, and transformers, among others, causes the DSL signal to distort. The distortion band extends from 0 Hz up to several times the maximum frequency of the DSL signal. Signal distortion occurs primarily at customer premises as it is much more likely to find DSL equipment in close proximity to POTS devices at customer locations rather than at central offices.
Since the distortion band spans from 0 Hz to a frequency greater than the maximum frequency band used in a DMT system, distortion adversely affects both POTS and DSL signals. Distortion in the POTS band causes discernible noise in the voice band and disturbs telephone conversations. Distortion in the DSL band reduces the signal to noise ratio across the full range of upstream and downstream spread spectrum signals. As a result, DSL performance is adversely affected in both the upstream and downstream channels.
DSL signal distortion is acute for DMT based modems because of the higher peak to RMS ratio of a DMT modulated signal. Because a DMT signal is merely a sum of sinusoids each with a different frequency and phase, it is possible that all 32 upstream channel sinusoids could coincide resulting in an extreme signal amplitude. Distortion is almost always dependent upon signal amplitude. As a result, signals with a high peak to average ratio, such as DMT signals, suffer more from distortion effects than a signal designed with a lower peak to average ratio, e.g. a quadrature amplitude modulated (QAM) signal.
Therefore, it is desired to maximize performance in a dual POTS-DSL communications system using the DMT standard by minimizing signal distortion. Since nonlinear distortion is a function of signal power, a method is desired to adjust downstream signal power in real time to reduce distortion interference. However, the presence of high-pass filters in the DSL equipment prohibit the direct measurement of signal distortion in the POTS frequency band by DSL equipment. With no direct capability of measuring POTS band signal distortion, DSL systems designers have been constrained to indirect estimations of POTS band distortion.
A common solution to enable the measurement of POTS band distortion consists of adding a programmable bypass circuit to the customer premises hardware. This solution is considered undesirable as it adds to both the initial purchase and long-term operating expense of the dual POTSxe2x80x94DSL communication system.
Prior art methods of measuring distortion include one and two-sinusoids tests. The tests insert known test signals into a system and measure the output signal at distinct frequencies. The amplitude of the system response to the sinusoidal test signals at distinct frequencies is proportional to the coefficients of a polynomial model of the communication system. By measuring signal distortion at those frequencies, it is possible to estimate each of the polynomial coefficients. However, intermodulation power can be extremely small and is often overcome by thermal noise and the dynamic range of the system. Often the amplitude of the sinusoidal test signal falls below the noise threshold making it impossible to use this technique to measure distortion.
Accordingly, it is desired to provide a system and method that efficiently measures signal distortion in the DSL frequency band (a wideband signal) in order to. adjust downstream signal power to minimize distortion interference throughout both the POTS and DSL frequency bands without adding hardware to the communication system.
Certain objects, advantages and novel features of the invention will be set forth in part in the description that follows and in part will become apparent to those skilled in the art upon examination of the following or may be learned with the practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out in the appended claims.
To achieve the objects and advantages of the present invention, the present invention is directed to a system and a method for measuring signal distortion in the downstream DSL frequency band to estimate echo distortion in the POTS band. The innovative solution of the present invention measures and regulates signal distortion, including that of the POTS band, based on downstream-band signal processing of the upstream echo. More specifically, the present invention measures distortion caused by a probe signal operating at an image frequency to estimate signal distortion in the POTS band for the full bandwidth of the upstream signal. Once an estimate of the POTS band distortion is available, the method adjusts the transmit power level in order to minimize signal distortion. The distortion reduction method consists of adjusting the transmit power of a R-REVERB sequence, called R-LINE-PROBE in the ADSL-DMT standard, until the noise observed in the downstream band falls below a predetermined threshold. Having determined an optimum upstream signal power cutback value, the method then uses the relationship between the optimized upstream signal power cutback value to determine a downstream signal power cutback value to minimize signal distortion in the system.
The system and method of the present invention combines the following ideas to minimize signal distortion at the customer premises in a ADSL-DMT communication system. The system uses a quasi-random periodic signal as a probe signal to measure signal distortion. The system further uses the measured signal distortion to estimate signal distortion in the POTS frequency band. Based on the estimated signal distortion in the POTS frequency band and a predetermined acceptable level of distortion, the system adjusts the downstream power cutback value.
The system and method of the present invention has a number of advantages that will be briefly summarized below. The system benefits from improved accuracy of signal distortion measurements as a direct result of using a wide-band probe signal. The method is adaptive, in that an optimum signal cutback value is determined based on system parameters for each particular customer premises ADSL equipment. The method requires only addition and multiplication operations. As a result, the method is fast and easily implemented on a digital signal processor. Additionally, the method provides an easy derivation of the optimum downstream signal power cutback value.
Importantly, the system and method of the present invention does not require addition or improvement to existing hardware. The method does not require a time and software code consuming alignment of the customer premises receiver to the transmitted probe signal. Lastly, the system will minimize signal distortion from sources other than the customer premises POTS equipment. In this regard, the system will minimize signal distortion due to line drivers, receiver amplifiers, and transformers. The system will enhance signal quality at the customer premises by reducing distortion across the full range of POTS and ADSL-DMT frequencies.